tree-wide: do not try to use outputs with no scene_output

- check for valid scene_output in output_is_usable()
- change many "output != NULL" checks to use output_is_usable()
- remove one now-redundant separate check for valid scene_output

Fixes a crash at startup (with autoEnableOutputs=no) due to
dereferencing null scene_output in create_output_config() since:

7d7ece21d9
("output: suppress error when output position is unavailable")

Fixes: #3357

src/regions.c

@@ -53,7 +53,7 @@ regions_from_cursor(struct server *server)
 	struct wlr_output *wlr_output = wlr_output_layout_output_at(
 		server->output_layout, lx, ly);
 	struct output *output = output_from_wlr_output(server, wlr_output);
-	if (!output) {
+	if (!output_is_usable(output)) {
 		return NULL;
 	}